Processes are known for curing polyacrylate polymers having dual cure sites by employing quaternary ammonium salts, for example, as a curative. Exemplary of methods for curing polyacrylate polymers are the following three U.S. patents.
U.S Pat. No. 3,875,092 to Morris discloses acrylate rubbers having both halogen and carboxyl cure sites which are vulcanized using as the only curing agent a quaternary ammonium salt. The vulcanized compositions exhibit good physical properties and low press-cured and post-cured compression sets.
U.S. Pat. No. 3,912,672 to Morris et al discloses a cure system for dual cure site acrylate rubbers. The acrylate rubbers have both halogen and carboxyl cure sites and are vulcanized using an alkali metal salt of a carboxylic acid or an organo-phosphoric acid. The alkali metal salt may be sodium or potassium stearate. The vulcanization can also occur with the addition of a quaternary ammonium salt or an amine to catalyze the vulcanization.
U.S. Pat. No. 3,976,610 to Morris, et al, discloses acrylate rubbers having both halogen and carboxyl cure sites which are vulcanized using an alkali metal salt of a carboxylic or organo-phosphoric acid and an amine-blocked isocyanate. The alkali metal salt may be sodium or potassium stearate. Like the other polyacrylate vulcanizable compositions, these compositions exhibit low press-cured and post-cured compression sets.
None of the above mentioned patents describe a no-post-cure process and do not recognize a temperature threshold effect for polyacrylate rubbers. The following patent does briefly mention the possibility of a no-post-cure process.
U.S. Pat. No. 4,650,834 to Yagishita, et al, describes a vulcanizable elastomer containing an epoxy group as the cure site. The vulcanizable epoxy elastomer also includes a carboxylic acid having at least two carboxyl groups in the molecule. The curative for vulcanizing the elastomer may be either a quaternary ammonium or phosphonium salt. While this patent does mention the possibility of omitting a post-curing stage, none of the examples omit the post-cured stage.
These above references are exemplary of curing processes which require about a 15 to 20 hour post-cure time (even Yagishita, et al, employed a 16 hour post-cure). Acrylate rubbers exhibit favorable qualities of weatherability, high temperature serviceability, and good oil resistance. These qualities make the rubbers useful for automotive and non-automotive applications such as industrial and out-of-door applications. Their use is limited by the tendency of the vulcanizates to post-cure during use. This results in property change and in some cases in failure of the article. To overcome these problems, the acrylate rubber vulcanizates are purposely postcured, often as long as 24 hours or more, to obtain a more complete cure. This is shown by a reduced compression set. It would be of great advantage to the industry to reduce or eliminate the time required for post-cure. It is of further advantage that the acrylate compounds have long shelf stability. In the past, processing such as compression, injection, or transfer molding; steam autoclave; continuous vulcanization (CV) cure techniques, and the like, have been limited by slow cure and poor shelf stability. These problems are solved by the present invention.